Tandem junction (n–p^+-Si/ITO/WO_3/liquid) core–shell microwire devices for solar-driven water splitting have been designed, fabricated and investigated photoelectrochemically. The tandem devices exhibited open-circuit potentials of E_(∝) = −1.21 V versus E^0′(O_2/H_2O), demonstrating additive voltages across the individual junctions (n–p^+-Si E_(∝) = −0.5 V versus solution; WO_3/liquid E_(∝) = −0.73 V versus E^0′(O_2/H_2O)). Optical concentration (12×, AM1.5D) shifted the open-circuit potential to E_(∝) = −1.27 V versus E^0′(O_2/H_2O) and resulted in unassisted H_2 production during two-electrode measurements (anode: tandem device, cathode: Pt disc). The solar energy-conversion efficiencies were very low, 0.0068% and 0.0019% when the cathode compartment was saturated with Ar or H_2, respectively, due to the non-optimal photovoltage and band-gap of the WO_3 that was used in the demonstration system to obtain stability of all of the system components under common operating conditions while also insuring product separation for safety purposes

Ardo, Shane

Atwater, Harry A.

Coridan, Robert H.

Fountaine, Katherine T.

Lewis, Nathan S.

Shaner, Matthew R.

Caltech Authors - Main

	  
	  
	  
	  
	  
	  
	  
Supplemental	  Information	  for	  
Photoelectrochemistry	  of	  Core–Shell	  Tandem	  
Junction	  n-­‐p+-­‐Si/n-­‐WO3	  Microwire	  Array	  
Photoelectrodes	  	  
	  
★,¢Matthew R. Shaner, ★,¢Katherine T. Fountaine, ★Shane A. Ardo, ★Robert H. Coridan, 
♯,¢Harry A. Atwater*, ★,¢Nathan S. Lewis* 
★Division of Chemistry and Chemical Engineering, M/C 127-72 
♯Thomas J. Watson, Sr. Laboratories of Applied Physics, M/C 180-32 
¢Joint Center for Artificial Photosynthesis, M/C 132-80 
1200 E. California Blvd. 
California Institute of Technology 
Pasadena, CA 91125, USA 
	   	  
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Table	  S1:	  Refractive	  index	  source	  for	  light	  absorption	  modeling	  
Material	   n,k	  source	  
Si	   Aspnes1	  
WO3	   Ellipsometry	  (Figure	  S1)	  
ITO	   Ref.	  2	  
SiO2	   Palik3	  
	  
	   	  
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Table	  S2:	  Geometric	  parameters	  for	  the	  Si/WO3	  microwire	  structure	  in	  Figure	  1b	  
used	  for	  the	  optical	  modeling	  
Si	  Microwire	  diameter	  (d)	   1.8	  μm	  
Si	  Microwire	  height	  (h)	   80	  μm	  
Si	  Microwire	  pitch	  (p)	   7	  μm	  
Si	  substrate	  thickness	  (tSi)	   Infinite	  
SiO2	  boot	  height	  (hboot)	   20	  μm	  
SiO2	  boot	  thickness	  (tboot)	   100	  nm	  
SiO2	  base	  thickness	  (tbase)	   500	  nm	  
ITO	  thickness	  (tITO)	   50	  nm	  
WO3	  coating	  thickness	  (tWO3)	   500	  nm	  
WO3	  top	  thickness	  (tWO3top)	   2.3	  μm	  
	   	  
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Figure	  S1:	  Refractive	  index	  data	  for	  WO3.	  	  The	  real	  component	  of	  the	  refractive	  
index,	  n,	  was	  obtained	  by	  fitting	  ellipsometry	  data	  and	  extrapolating	  above	  the	  
bandgap	  with	  a	  Lorentzian	  function.	  	  Given	  the	  n	  obtained	  by	  ellipsometry,	  an	  
adapted	  form	  of	  a	  multilayer	  material	  transfer	  matrix	  program	  was	  used	  to	  fit	  
integrating	  sphere	  transmission	  data	  to	  extract	  the	  imaginary	  component	  of	  the	  
refractive	  index,	  k.	  
	   	  
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Figure	  S2:	  Concentration	  profile	  of	  boron	  in	  an	  n-­‐Si	  planar	  wafer	  after	  BCl3	  drive	  in,	  
from	  a	  secondary	  ion	  mass	  spectrometry	  (SIMS)	  measurement	  (see	  experimental	  
section).	  	  This	  shows	  an	  emitter	  thickness	  of	  ~250	  nm.	  
	   	  
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Figure	  S	  3:	  a)	  SEM	  image	  captured	  by	  the	  EDX	  instrument	  of	  a	  tandem	  microwire	  
cross	  section.	  	  The	  line	  scan	  performed	  is	  overlaid	  on	  this	  image	  with	  the	  beginning	  
and	  end	  indicated.	  b)	  Line	  scan	  data	  as	  reported	  by	  the	  Oxford	  Aztec	  software.	  	  It	  
shows	  a	  clear	  transition	  from	  a	  Si	  rich	  area	  (<0.35	  µm)	  to	  an	  indium	  rich	  area	  (0.35	  
µm-­‐0.45	  µm)	  to	  a	  W	  and	  O	  rich	  area	  (>0.45	  µm).	  	  This	  further	  verifies	  the	  presence	  
of	  the	  layered	  structure	  as	  described	  in	  the	  main	  text.	  
	   	  
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	   	  Figure	  S	  4:	  (a),	  (c),	  (e)	  SEM	  images	  captured	  by	  the	  EDX	  software	  that	  indicated	  the	  
point	  where	  data	  was	  taken	  at.	  	  (b),	  (d,)	  (f)	  Corresponding	  EDX	  data	  the	  points	  
depicted	  in	  (a),	  (c),	  (e.)	  	  These	  data	  also	  confirm	  the	  layered	  structure	  described	  in	  
the	  text.	  
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Figure	  S	  5:	  Planar	  single	  (WO3/liquid)	  and	  tandem	  (n-­‐Si/p+-­‐Si/ITO/WO3/liquid)	  
junction	  current	  density	  vs.	  potential	  behavior	  in	  1.0	  M	  H2SO4	  under	  one	  Sun	  
illumination	  (AM	  1.5G).	  
	   	  
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Figure	  S6:	  Effect	  of	  concentrated	  illumination	  on	  tandem-­‐junction	  
WO3/Si	  microwire	  photoelectrochemical	  performance	  in	  1.0	  M	  H2SO4.	  	  
Data	  shown	  are	  for	  the	  potential	  sweep	  at	  20	  mV-­‐s-­‐1.	  
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Figure	  S7:	  Two-­‐cell,	  three-­‐	  or	  two-­‐electrode	  evaluation	  set-­‐up	  with	  the	  cathode	  
compartment	  on	  the	  left	  and	  anode	  compartment	  on	  the	  right.	  	  The	  cathode	  
compartment	  is	  sealed,	  continually	  purged	  with	  an	  Ar	  or	  H2	  gas	  stream,	  and	  
contains	  ports	  for	  the	  Pt	  disc	  electrode	  and	  Pt	  mesh	  counter	  electrode.	  	  The	  anode	  
compartment	  contains	  the	  tandem	  junction	  electrode	  illuminated	  through	  a	  quartz	  
window,	  an	  SCE	  reference	  electrode,	  and	  a	  Pt	  mesh	  counter	  electrode.	  	  The	  cells	  are	  
separated	  by	  a	  Nafion	  membrane	  sandwiched	  between	  flanges	  on	  the	  sides	  of	  the	  
cells	  and	  held	  together	  with	  a	  clamp.	  
	  
	  
	  
	  
	  
	  
	  
	  
	  
	  
	  
	  
	  
	  
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Figure	  S8:	  Mass	  spectrometry	  data	  for	  H2	  detection	  at	  a	  Pt	  disc	  electrode.	  
	   	  
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Figure	  S9:	  Structure	  used	  to	  model	  the	  absorption	  in	  the	  1D	  optoelectronic	  model.	  	  
Each	  junction	  (n-­‐p+-­‐Si	  and	  n-­‐WO3/liquid)	  was	  modeled	  individually.	  
	   	  
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Figure	  S10:	  Modeled	  light	  (1	  sun)	  and	  dark	  J–E	  behaviour	  for	  (a)	  an	  n-­‐p+-­‐Si	  
homojunction	  and	  (b)	  an	  n-­‐WO3/Eo’(O2/H2O)	  liquid	  junction.	  
	   	  
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Figure	   S11:	   Experimental	   (green)	   and	   modeled	   (blue)	   hydrogen	   evolution	   J-­‐E	  
behaviour.	  	  The	  experimental	  data	  includes	  solution	  and	  mass	  transport	  resistances	  
that	  were	  not	  included	  in	  the	  modeled	  data.	   	  Thus	  the	  modeled	  data	  represents	  an	  
ideal	  system	  where	  solution	  and	  mass	  transport	  resistances	  are	  absent.	  
	   	  
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Figure	  S12:	  Carrier	  generation	  rate	  map	  for	  four	  wavelengths:	  (a)	  350	  nm,	  (b)	  400	  
nm,	  (c)	  450	  nm,	  and	  (d)	  500	  nm.	  	  	  
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References	  
	  
	  
	  
(1)	   D.	  E.	  Aspnes.	  Optical	  properties	  of	  Si.	  In	  Hull,	  editor,	  Properties	  of	  Crystalline	  
Silicon,	  page	  677.	  INSPEC,	  IEE,	  London	  UK,	  1999.	  	  
(2)	   Dobrowolski,	  J.A.;	  Li,	  L.;	  Hilfiker,	  J.N.	  Applied	  Optics	  38,	  22	  (1999).	  
(3)	   E.	  D.	  Palik.	  Handbook	  of	  Optical	  Constants	  of	  Solids.	  New	  York,	  Academic	  
Press,	  1997.	  	  
	  
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English

Photoelectrochemistry of core–shell tandem junction n–p^+-Si/n-WO_3 microwire array photoelectrodes

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Photoelectrochemistry of core–shell tandem junction n–p^+-Si/n-WO_3 microwire array photoelectrodes

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